With the advent of high speed data communications, optical transmission media are becoming increasingly common. For high bandwidth applications, for example, fiber optic cable and hybrid fiber-coaxial cable are often the transmission media of choice. The well-known advantages of such optical transmission systems include high bandwidth, high transmission capacity, and high noise immunity, resulting in very high data transmission speeds and low error rates.
The use of optical transmission media has also engendered a need for optical switching devices, optical routing devices, and optical logic gates. Typically in the prior art, the data or other information for optical transmission has been interconverted between optical and electronic forms. For example, data for transmission is typically generated and packaged electronically, then converted into laser pulses for optical transmission. Again, at the receiving end, the received light pulses are converted into an electronic form for further processing in, for example, a personal computer, a workstation, a server, or a router.
Various optical devices have been developed, however, which diminish the need for such optical-electronic interconversions. For example, an optical device referred to as a "TOAD" has been developed which performs the logical AND function. As another example, this can be used to implement an optical demultiplexer, which allows for selection of one or more channels from time division multiplexed channels, with one channel selected per optical AND gate. These optical gates typically utilize a clock pulse periodically ANDed with selected bits of an optical time division multiplexed ("OTDM") channels to produce an output bit stream from the selected channel. See, e.g., Prucnal et al. U.S. Pat. No. 5,493,433, "Terahertz Optical Asymmetric Demultiplexer". For high speed data transmission, for example, an OTDM transmission in the range of terabits per second may include 1000 multiplexed channels, utilizing a transmission frame of 1000 bits with one bit per channel, with each channel operating at a 1 gigabit per second data transmission rate, with a bit period (or bit time) of 10.sup.-12 seconds.
To enable such extremely high data rates in optical communications, however, various difficulties must be overcome which do not arise in lower speed communications. For example, due to external or environmental factors, the physical characteristics of optical fiber may change slightly, such as changes in the index of refraction and the physical length expanding or contracting depending upon environmental temperatures. At lower data rates, such changes may be inconsequential. At these higher terabit per second data rates, however, such physical changes may affect several bit periods, requiring new solutions to these new, heretofore non-existent problems. For example, due to the physical characteristics of optical fibers, thermal fluctuations may cause a relative drift, skew or other mismatch between the clock pulse and the desired OTDM data channel in a demultiplexer. At terabit data rates, such a drift or skew may cause errors in data transmission, such as erroneously interpreting a logical 1 as a logic zero, or may cause the wrong channel to be erroneously decoded, particularly should the drift span several bit periods. In contrast, such skewing in the picosecond range is irrelevant at current lower speed data rates, such as gigabit/second rates.
As a consequence, a need remains for an apparatus and method to maintain bit synchronization in optical transmission media. Such an apparatus and method should be able to track relative bit drifting or skewing, and maintain bit synchronization spanning several bit periods during a single transmission and without interruption of the transmission. In addition, such an apparatus and method should be capable of cost-effective implementation.